The interface to a color picking service. This service provides other sub-compnents the ability to request that the user pick a color.
Colors are represented as CSS color value strings. I.E.: #f7d87e.
See the very similar SizePicker service for a more detailed discussion of
how this is structured.
Create a reqeust for the color picker service to ask the user for a color.
When the user has chosen a color, it will be used with the supplied function
to create a message object, which will be routed to your update function.
The supplied string is used to identify the request to the user.